Making the Silverlight TreeView bindable two-way

private HelpTopic selectedTopic;
public HelpTopic SelectedTopic
{
    get { return selectedTopic; }
    set
    {
        if (selectedTopic == value)
        {
            return;
        }
        selectedTopic = value;
        OnPropertyChanged("SelectedTopic");
    }
}

<ListBox ItemsSource="{Binding HelpTopics}" DisplayMemberPath="Name" 
         SelectedItem="{Binding SelectedTopic, Mode=TwoWay}" />
<TextBlock Text="{Binding SelectedTopic.Name}" />

<StackPanel>
    <StackPanel Orientation="Horizontal">
        <ListBox ItemsSource="{Binding HelpTopics}" DisplayMemberPath="Name" 
                 SelectedItem="{Binding SelectedTopic, Mode=TwoWay}" Width="300" 
                 HorizontalAlignment="Stretch" />
        <slt:TreeView VerticalAlignment="Stretch" ItemsSource="{Binding HelpTopics}" 
                      SelectedItem="{Binding SelectedTopic, Mode=TwoWay}" Width="300">
            <slt:TreeView.ItemTemplate>
                <slt:HierarchicalDataTemplate ItemsSource="{Binding SubTopics}">
                    <TextBlock Text="{Binding Name}" VerticalAlignment="Center" />
                </slt:HierarchicalDataTemplate>
            </slt:TreeView.ItemTemplate>
        </slt:TreeView>
    </StackPanel>
    <Border BorderBrush="Black" BorderThickness="1" HorizontalAlignment="Stretch">
        <StackPanel Orientation="Horizontal">
            <TextBlock Text="You selected: " Margin="4" />
            <TextBlock Text="{Binding SelectedTopic.Name}" VerticalAlignment="Center" Margin="4" />
        </StackPanel>
    </Border>
</StackPanel>

public class SelectionService
{
    public static readonly DependencyProperty SelectedItemProperty = 
           DependencyProperty.RegisterAttached("SelectedItem", typeof(object), typeof(SelectionService), 
           new PropertyMetadata(null, OnSelectedItemChanged));

    public static void SetSelectedItem(DependencyObject o, object propertyValue)
    {
        o.SetValue(SelectedItemProperty, propertyValue);
    }
    public static object GetSelectedItem(DependencyObject o)
    {
        return o.GetValue(SelectedItemProperty);
    }

    private static void OnSelectedItemChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        TreeView treeView = d as TreeView;
        if (treeView == null)
        {
            return;
        }
        TreeViewItem item = treeView.ItemContainerGenerator.ContainerFromItem(e.NewValue) as TreeViewItem;
        if (item == null)
        {
            return;
        }
       item.IsSelected = true;
    }
}

<slt:TreeView ... local:SelectionService.SelectedItem="{Binding SelectedTopic}">

private static void OnSelectedItemChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
    TreeView treeView = d as TreeView;
    if (treeView == null)
    {
        return;
    }

    TreeViewItem item = treeView.ItemContainerGenerator.ContainerFromItem(e.NewValue) as TreeViewItem;
    if (item != null)
    {
        item.IsSelected = true;
        return;
    }

    for (int i = 0; i < treeView.Items.Count; i++)
    {
        SelectItem(e.NewValue, treeView.ItemContainerGenerator.ContainerFromIndex(i) as TreeViewItem);
    }
}

private static void SelectItem(object o, TreeViewItem parent)
{
    if (parent == null)
    {
        return;
    }

    bool isExpanded = parent.IsExpanded;
    if (!isExpanded)
    {
        parent.IsExpanded = true;
        parent.UpdateLayout();
    }
    TreeViewItem item = parent.ItemContainerGenerator.ContainerFromItem(o) as TreeViewItem;
    if (item != null)
    {
        item.IsSelected = true;
        return;
    }

    for (int i = 0; i < parent.Items.Count; i++)
    {
        SelectItem(o, parent.ItemContainerGenerator.ContainerFromIndex(i) as TreeViewItem);
    }

    if (parent.IsExpanded != isExpanded)
    {
        parent.IsExpanded = isExpanded;
    }
}